The invention relates to hydrolysis resistant polyester polyurethane elastomers. More particularly, the invention relates to polyurethane elastomers for use in footwear, especially shoe soles.
Polyester polyurethanes for applications such as footwear should have excellent flexural properties and hydrolysis resistance. Polyester polyurethane elastomers made with aromatic ester polyols such as o-phthalic acid polyester polyols have excellent hydrolysis resistance. However, these elastomers have poor flexural properties. Polyester polyurethanes made with aliphatic polyester polyols such as adipic acid polyester polyols have excellent flexibility, but poor hydrolysis resistance.
Polyester polyurethane elastomers made with aliphatic polyester polyols such as adipic acid based polyols have employed additives such as caprolactone polyols and carbodiimides such as Staboxyl I in an attempt to achieve improved properties. Caprolactone polyols, however, are undesirably expensive. Use of carbodiimides such as Staboxyl I do not produce polyester polyurethane elastomers which have satisfactory hydrolysis resistance as gauged by retained tensile strength after aging in humid environments.
Retained tensile strength of polyester polyurethanes made with aliphatic polyester polyols which include Staboxyl I is only about 60-70% of the original tensile strength after humid aging at 70xc2x0 C. and 100% relative humidity for seven days. For applications wherein the polyester polyurethanes are employed as shoe soles, the retained tensile strength is desirably about 75-90% of the tensile strength prior to aging.
A need therefore exists for polyester polyurethane formulations which can be used to produce polyurethanes which show improved hydrolysis resistance and which avoid the disadvantages of the prior art.
The invention relates to polyurethanes having improved hydrolysis resistance and tensile strength retention. The polyurethanes are made by reacting an isocyanate terminated prepolymer with a polyol component which includes a polyester polyol blend made from an aliphatic carboxylic acid based polyester polyol, preferably adipic acid based polyester polyol, and an aromatic carboxylic acid based polyester polyol, preferably o-phthalic acid based polyester polyol.
More specifically, the polyurethane elastomers are made by reacting a first component comprising a polyol component having a blend of an aromatic dicarboxylic acid based polyester polyol, and an aliphatic dicarboxylic acid based polyester polyol, a blowing agent and preferably, a chain extender, with an isocyanate terminated prepolymer. The aromatic dicarboxylic acid based polyester polyol is the reaction product of an aliphatic alcohol and ortho-phthalic acid. The aliphatic alcohol is any of ethylene glycol, diethylene glycol, hexanediol, and neopentyldiol. The aliphatic dicarboxylic acid is any of adipic acid, succinic acid, glutaric acid, and suberic acid. The aromatic polyester polyol is present in the polyol blend in an amount of about 12 wt. % to about 26 wt. %, preferably about 15 wt. % to about 20 wt. %, based on the weight of the polyol blend, remainder aliphatic polyester polyol. In a preferred aspect, the ortho-phthalic acid reacted with the aliphatic alcohol has less than about 10 wt. % total of tere-phthalic acid and isophthalic acid.
The aliphatic dicarboxylic acid based polyester polyol is the reaction product of adipic acid and glycol or glycols such as ethylene glycol, diethylene glycol, and 1,4-butanediol, and has an OH value of about 36 to about 56. The blowing agent is any of water, acetone, pentane, hexane, cyclopentane, (1,1,1,2-tetrafluoroethane), (1,1,1,3,3-pentafluoropropane), (1,1,2,2,3-pentafluoropropane), (1,1,1,2,3,3-hexafluoropropane), and methylene chloride, preferably water in an amount of about 0.02% to about 1.2% by weight based on the total weight of the polyol component.
In a more preferred aspect, the polyurethane elastomers of the invention are the reaction products of a first component comprising a polyol component that includes a polyol blend formed of an aliphatic polyester polyol and an aromatic polyester polyol, with a second component that is a diphenylmethane diisocyante prepolymer having about 15% NCO to about 24% NCO. The aliphatic polyester polyol is the reaction product of adipic acid with diethylene glycol and ethylene glycol. The aromatic polyester polyol is the reaction product of diethylene glycol with ortho-phthalic acid. The aromatic polyester polyol is present in an amount of about 15 wt. % to about 20 wt. % of the polyol blend, remainder aliphatic polyester polyol. The polyol component also includes 1,4 butanediol as a chain extender and water as a blowing agent.
Having summarized the invention, the invention is described in detail below by reference to the following detailed description and non-limiting examples.